Magnetic stereophonic phonograph pickup

ABSTRACT

A magnetic stereophonic pickup having four coil carrying core members arranged symmetrically around the axis of an annular ceramic magnet against the inner pole and end face of which one end of said core members abut the opposite ends being magnetically interconnected. A magnetic armature connected to a stylus is so supported within and coaxial with the magnet that it can oscillate in any direction about a point adjacent the outer end thereof. The armature acts as a variable magnetic load converting stylus movements into variations of the magnetic potentials at the magnet abutting ends of the core members, said potential variations in turn causing flux variations in the coil carrying core members.

This invention relates to electromagnetic transducers for reproductionof stereophonic phonograph records.

More particularly the invention relates to such transducers having amagnetic structure defining an airgap and comprising four magnetic legseach surrounded by its respective coil as well as permanent magneticmeans for producing a magnetic field in said airgap in which a styluscarrying armature is pivotally mounted. By arranging the magnetic legsin two pairs perpendicular to each other the signals produced bymovements of the stylus can be decomposed into two components eachcorresponding to its respective recording track of a stereophonicrecord.

In the early pickup assemblies of this type the armature itself isconstituted by a permanent magnet. However, the still increasingrequirements as to reproduction of signals at the upper end of the tonefrequency spectrum has led to the development of pickups in which theeffective stylus mass has been reduced by using a permanent magnetplaced outside the oscillating stylus system for producing themagnetizing force, so that the armature may be constituted by a lighttube of magnetically conducting material such as soft iron.

The known pickups of this kind have the feature in common with othermagnetic pickup assemblies that the armature is coupled magnetically inseries with the coil carrying legs, so that the signal producing fluxchanges are produced by alterations of the overall reluctance of themagnetic circuit caused by movements of the armature.

According to the present invention a novel structure is provided inwhich the armature is so placed that it forms a magnetic load coupled inparallel to the permanent magnet or magnets, meaning that the armatureand the magnet or magnets form a magnetic circuit independently of thefour coil carrying legs which form two magnetic conductors each havingboth ends engaging the same pole face of said magnet or pole faces ofthe same polarity of said magnets. In the rest position of the armaturein which the structure is perfectly symmetrical both ends of eachmagnetic conductor are at the same magnetic potential and hence no fluxis flowing through the coil carrying legs. When the armature isdeflected, however, the magnetic load on the permanent magnet portionopposite one end of a magnetic conductor is increased while the load onthe magnet portion opposite the other end of said conductor is decreasedresulting in the occurrence of a magnetic potential difference betweenthe ends of the magnetic conductor whereby a flux is produced in thecoil carrying legs.

An object of the invention is to provide an improved electromagnetictransducer in which the effective mass of the oscillating stylus systemis reduced by placing the permanent magnet or magnets outside saidsystem.

It is a further object of the invention to provide a transducer in whichthe magnetic structure including the armature is perfectly symmetrical.

A still further object of the invention is to provide a magneticstructure for use in a transducer in which the coil carrying legs can bemade very thin thus allowing the use of signal coils having lowresistance and capacity.

Another object of the invention is to provide an electromagnetictransducer which is easy and cheap to produce.

Still another object of the invention is to provide a magnetic stereopickup having improved sound reproduction characteristics, particularlyat the upper end of the tone frequency range.

Other objects and advantages will appear from the following descriptionwhen taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a longitudinal section of an embodiment of the pickupaccording to the invention,

FIG. 2 is a section along the line II--II in FIG. 1,

FIG. 2A shows a monophonic variation of the section shown in FIG. 2,

FIG. 3 is a perspective view of the front end of the magnetic system andthe stylus assembly mounted therein,

FIGS. 4 and 4a show schematically one half of the magnetic system withthe armature in the neutral and deflected position, respectively, and

FIGS. 5 and 5A are similar schematics showing a second embodiment of themagnetic system in which the armature is constituted by a permanentmagnet.

The pickup shown in FIGS. 1-3 comprises a housing 1 cast in a suitableplastics material. At one end the housing has a recess 7 in which ismounted a contact member 2 with four legs 3, of which only two are shownin FIG. 1. At the other end of the housing is provided a replaceablestylus assembly comprising a cast plastic support body 4 which, by meansof a projecting hollow guide pin 5 of non-circular cross-section,engages a recess 6 formed in the housing 1 and having the samecross-section.

Between the bottom of the recess 7 and the front face of the housing 1is provided a through-going recess with a rear portion 8 ofsubstantially square cross-section and a front portion 9 thecross-section of which, as indicated in FIG. 2, is formed as a middlecircle and four peripheral circles tangential to the middle circle andequally spaced along the periphery thereof.

The said through-going recess 8, 9 serves as accommodation for themagnetic system which comprises four cylindrical legs of permalloy, thefront portions of which are disposed in their respective cylindricaledge portions of the recess 9 and extend rearwardly through the recess8. The rear ends of the legs 10 are attached to and interconnectedmagnetically by a cross member 11, which is likewise made of permalloy.About each leg 10 is disposed a coil 12 which is mounted in the recess8, and each pair of diametrically opposed coils are connected to oneanother and to two of the legs 3 by leads 13. On the front ends of thelegs 10 and in extension thereof is mounted an annular permanent magnet14 which is magnetized in an axial direction and consists of a ceramic,magnetically anisotropic material. The magnet 14 is disposed in anenlarged section of the recess 9 at the front of the housing 1.

In the lower portion of the support body 4 of the stylus assembly isformed an inclined cylindrical, through-going channel 15 opening in theunder face of the body and forming an elongation of the recess 9 in thehousing 1. In this channel is secured a tube 16 of a non-magneticmaterial, for instance aluminium which projects from the support body 4and, when the stylus assembly is fitted, enters the cylindrical centralportion of the recess 9.

Within the tube 16 a tubular soft iron armature 17 is mounted by meansof an annular, resilient pad 18, for instance of rubber or neoprene, tobe freely pivotable in all directions about a point disposed in oradjacent to the center of the pad. The armature 17 is slightly longerthan the annular magnet 14 and is disposed co-axial with andsymmetrically with respect to the magnet. To the armature is attachedone end of a tubular, thin-walled aluminium stylus arm 19 extendingthrough the channel 15. The free end of the stylus arm 19 projectingoutwardly of the support body 4 is flattened and supports a stylus 20.

On the under face of the housing 1 is mounted a magnetic screen 21 whichextends upwardly along the front face of the housing between the housingand the support body 4 of the stylus assembly.

To illustrate the operation of the pickup FIG. 4 presents a schematicview of one half of the electromagnetic system corresponding to onesound channel of a stereo recording. The oscillation point of thearmature is here designated 22. The anisotropic ceramic annular magnet14 shows great reluctance in all directions perpendicular to the axialdirection and is therefore magnetically equivalent to two permanent rodmagnets 14a and 14b disposed at the end and in extension of theirrespective legs 10 as indicated in FIGS. 4.[.A.]. and4.[.B.]..Iadd.a.Iaddend., in which the said magnets 14a and 14b arepresumed to have their north pole at the leg end. Assuming the twomagnets 14a and 14b to be of equal strength, the magnetic conductorformed by the legs 10 and the cross member 11 will have the samemagnetic potential at its two end faces in the shown neutral position ofthe armature 17, and no flux will therefore flow through this conductor.But a relatively high flux will pass from the north pole of each magnetthrough the armature to the south pole of the magnet, as indicated bythe thin lines, and as the armature has a much lower reluctance than theceramic magnets 14, 14a and 14b and the airgaps in actual practice arevery small, this flux may with some justification be described as ashort circuit flux, since a sufficient magnetic load is imparted to thepermanent magnet that the magnetic potentials of the pole faces thereofare substantially reduced.

If the armature as a result of the stylus movement oscillates downwardsin the plane of the paper about the point 22 as shown in FIG. 4a it willapproach the lower magnet and thereby reduce the reluctance in the shortcircuit flux path for said magnet and simultaneously move away from theupper magnet, with the result that the short circuit flux thereof meetsincreased reluctance. In other words, the lower short circuit flux willbe increased and the upper short circuit flux will be reduced. Onaccount of the relatively high inherent reluctance of the magnets thiswill cause the magnetic potential at the north pole of the lower magnetto be reduced, while the magnetic potential at the north pole of theupper magnet will be increased. The combined effect of the increasedmagnetic potential of the upper magnet and the decreased reluctance ofthe lower air gap will result in a flux in the magnetic conductor 10,11, 10 as illustrated by a single flux line f in FIG. 4a, inducing inthe coil windings 12 electrical voltages which added together representthe sensed sound signal.

Armature oscillations resulting from the other sound channel follow adirection perpendicular to the plane of the paper and are converted inthe same manner to electrical voltages in the second half of theelectromagnetic system disposed in this plane.

The magnetic conductor 10, 11, 10 leads only the effective signal fluxwhich changes direction in response to the armature oscillations, andthe legs 10 can therefore be designed very thin without risking that themagnetic point of operation deviates from the rectilinear part of themagnetization curve.

The stereo pickup described above and shown in FIGS. 1-3 of the drawingscan be changed into a mono pickup, as shown in FIG. 2A, merely byremoving one pair of legs 10 and rotating the remaining pair an angle ofapproximately 45° about the common axis of the magnetic structure withrespect to the stylus assembly.

In the embodiment described the armature 17 is constituted by athin-walled tube of soft iron, by which the effective inertia of theoscillating stylus assembly is minimized. In FIGS. 5 and 5A, however, isschematically shown the magnetic circuit of an embodiment in which thearmature 17 is constituted by a permanent magnet which may be tubular orrod shaped and which is poled oppositely to the fixed, outer magnet 14of magnets 14a and 14b, so that it increases the short circuit flux. Inthis way the sensitivity of the pickup can be increased to some degree.

It will be understood that the invention is not limited to theembodiments shown and described and that it will be possible within thescope of the invention to modify the structure in various respects. Thusfor example the legs 10 may be square or rectangular in cross-sectioninstead of circular as also the armature may have other cross-sectionshapes.

What we claim is:
 1. A magnetic structure for a stereophonic pickup inthe form of an electromechanical transducer, said structure comprising ahollow cylindrical magnet made of a magnetizable ceramic material andhaving magnetic poles of opposite polarity at the end faces thereof,four elongated core members abutting one end face of said magnet andextending therefrom in a direction parallel to the axis thereof, saidcore members being arranged symmetrically with respect to said axis,means for magnetically interconnecting said core members at the endsthereof remote from said magnet, further comprising a stylus carrying.Iadd.permanent magnet .Iaddend.armature, and means supporting said.Iadd.permanent magnet .Iaddend.armature within and coaxial with saidhollow cylindrical magnet for pivotal movement in all directions about apoint of said axis spaced from .[.the center point.]. .Iadd.any pointlocated interiorly .Iaddend.of said magnet.Iadd., said permanent magnetpoled oppositely to the polarity of said hollow cylindricalmagnet.Iaddend..
 2. An electromagnetic transducer comprising incombination: a housing having a bore extending obliquely from onesurface thereof, an annular permanent magnet of a ceramic materialsupported within said housing coaxially with and surrounding said bore asmall distance from the outer end thereof, a ring of elastic materialmounted within and engaging the walls of said bore adjacent .Iadd.to butoutside .Iaddend.the outer end of said magnet, a tubular .Iadd.permanentmagnet .Iaddend.armature supported by said ring and extending coaxiallywith said .Iadd.angular .Iaddend.bore and said magnet beyond the innerend of the latter, a stylus arm fixed to said .Iadd.permanent magnet.Iaddend.armature and carrying a stylus on its outer end extendingbeyond said surface of said housing, four rods of a high permeancematerial .[.and with circular cross-section.]. extending parallel to andequiangularly spaced around the axis of said bore, said rods abuttingthe inner end face of said magnet with their outer ends and extendingbeyond the inner end of said bore, a cross-piece of magnetizablematerial magnetically interconnecting the inner ends of said rods andfixedly held in said housing, and four coils mounted on the innerportions of said rods respectively between the inner end of said boreand said cross-piece, .[.sand.]. .Iadd.said .Iaddend.coils being seriesconnected in pairs each consisting of oppositely disposed coils andconnected to respective terminals.Iadd., said permanent magnet having apolarity opposite to that of said annular permanent magnet.Iaddend.. 3.An electromechanical transducer comprising a magnetic structure definingan airgap .[.and.]. symmetrically disposed with respect to an axistherein, a stylus carrying armature .Iadd.comprising a permanent magnetand .Iaddend.extending through said airgap and means pivotally mountingsaid armature around a pivot located on said axis adjacent .Iadd.to butoutside .Iaddend.one end of the airgap, said magnetic structurecomprising magnetic means surrounding the armature and having magneticpoles of one polarity adjacent one end of the armature and magneticpoles of the opposite polarity adjacent the other end of the armature,.Iadd.said permanent magnet poled opposite to the polarity of saidmagnetic means, .Iaddend.two pairs of core members symmetricallydisposed with respect to the axis of the airgap and defining planesforming a right angle, said core members abutting magnetic pole faces ofthe same polarity at one end of said magnetic means, means formagnetically interconnecting the core members of each of said pairs atthe ends thereof remote from said pole faces, and a coil mounted on eachof said core members.
 4. An electromechanical transducer as claimed inclaim 3, in which the said magnetic means comprises a hollow cylindricalbody made of a magnetizable ceramic material. .[.5. An electromechanicaltransducer as claimed in claim 3 in which said armature comprises apermanent magnet..].
 6. An electromagnetic transducer comprising:a pairof permanent magnet means arranged in spaced parallel relationship;magnetic conductor means magnetically connecting two pole faces of saidpermanent magnet means having the same polarity, said two pole facesbeing connected to said magnetic conductor means; coil means surroundinga portion of said magnetic conductor means for generating an electricalcurrent in response to a magnetic flux induced in said coil means bysaid permanent magnet means; an armature .Iadd.comprising a furtherpermanent magnet.Iaddend.; a stylus connected to said armature; andmounting means pivotally mounting said armature .Iadd.for pivoting abouta point located outside said permanent magnet means but.Iaddend.magnetically in parallel with said permanent magnet means tovary a magnetic load imparted to said permanent magnet means by saidarmature.Iadd., said permanent magnet poled opposite to the polarity ofsaid annular cylindrical magnet.Iaddend..
 7. An electromagnetictransducer as defined in claim 6, further comprising:a further pair ofpermanent magnet means arranged in spaced parallel relationship about acommmon longitudinal axis with said pair of permanent magnet means;further magnetic conductor means magnetically connecting further twopole faces of said further pair of permanent magnet means having thesame polarity, said further two pole faces being connected to saidfurther magnetic conductor means; and further coil means surrounding aportion of said further magnetic conductor means for generating anelectrical current in response to a magnetic flux induced in saidfurther coil means by said further permanent magnet means; wherein therespective planes defined by said pair of permanent magnet means andsaid further pair of permanent magnet means form substantially rightangles with each other and said mounting means mounts said armature forpivotal movement in all directions.
 8. An electromagnetic transducer asclaimed in claim 7, wherein:said permanent magnet means comprises fourbar magnets arranged symmetrically and equiangularly spaced around saidcommon longitudinal axis; said magnetic conductor means comprises fourrod-shaped members of a high permeance material and a common cross-piecemagnetically interconnecting respective one ends of said four rod-shapedmembers, said four bar magnets connected to and forming extensions ofthe opposite ends of respective ones of said rod-shaped members; andsaid coil means and further coil means together comprise two pairs ofseries connected coil pairs oppositely disposed on portions of saidrod-shaped members.
 9. An electromagnetic transducer having a magneticstructure comprising an annular cylindrical magnet of a ceramic materialmagnetized in an axial direction and having a plurality of angularlyspaced portions, magnetic conductor means, portions of which aresurrounded by a coil, interconnecting a first pair of said angularlyspaced portions, a stylus carrying armature .Iadd.comprising a furtherpermanent magnet.Iaddend., means mounting said armature between saidfirst pair of angularly spaced portions for pivotal movement .Iadd.abouta point external of said annular cylindrical magnet .Iaddend.in responseto stylus movements in the plane of said first pair of spaced portionsto cause variations in the magnetic load imparted to said spacedportions by said armature, said magnetic conductor means interconnectingtwo pole faces of the same polarity of said first pair of angularlyspaced portions.
 10. An electromagnetic transducer as defined in claim9, further comprising a second magnetic structure having a second pairof angularly spaced portions arranged in spaced parallel relationshipand interconnected by a second magnetic conductor, portions of which aresurrounded by a second coil, wherein the planes of said first and secondpairs of spaced portions form substantially right angles with respect toeach other and wherein said first and second magnetic structures have acommon longitudinal axis, said armature being mounted for pivotalmovement in all direction between said first and second pairs of spacedportions.
 11. An electromagnetic transducer as claimed in claim 10 inwhich said first and second magnetic conductors comprise four parallelrods of a high permeance material arranged symmetrically andequiangularly spaced around the axis of said cylindrical magnet with oneend of said rods abutting one end face thereof, the opposite ends ofsaid rods being magnetically interconnected by a common cross piece. 12.An electromagnetic transducer as claimed in claim 11 in which said rodsare circular in cross-section.
 13. An electromechanical transducer,comprising:a pair of permanent magnet portions having an airgap betweenthem and providing respective components of magnetic flux similarlydirected in said airgap; yoke means magnetically connecting pole facesof like polarity of each said pair of permanent magnet portionstogether, said pole faces being connected to said yoke means; electricaloutput coil means surrounding portions of said yoke means for generatingelectrical output signals induced by magnetic flux changes in said yokemeans; an armature .Iadd.including a further permanent magnet.Iaddend.;and mounting means pivotally mounting said armature .Iadd.for pivotingabout a pivot point external of said permanent magnet portions and.Iaddend.magnetically in parallel with said pair of permanent magnetportions to provide a path for said respective .[.components of magneticflux, to vary the magnetic.]. potentials of said pole faces and tothereby produce said flux changes in said yoke means.
 14. Anelectromechanical transducer as defined in claim 13, furthercomprising:a further pair of permanent magnet portions arranged onopposite sides of said airgap and angularly spaced substantially 90°from said pair of permanent magnet portions; wherein said yoke meansmagnetically connects further pole faces of like polarity of saidfurther pair of permanent magnet portions; further electrical outputcoil means surrounding further portions of said yoke means forgenerating further electrical output signals induced by magnetic fluxchanges in said further portions of said yoke means; said mounting meansmounting said armature for pivotal movement in all directions about.[.a.]. .Iadd.said .Iaddend.pivot point on the central axis of saidairgap and spaced therealong from a central point of symmetry of saidpair and further pair of permanent magnet portions.
 15. Anelectromechanical transducer as defined in claim 14, wherein saidpermanent magnetic portions comprise respective angularly spacedintegral parts of a hollow cylindrical body of a magnetizableanisotropic material and having its direction of maximum permeabilitysubstantially parallel to its axis, said body being permanentlymagnetized in said direction, and surrounding said airgap. .[.16. Anelectromechanical transducer as defined in claim 14, wherein saidarmature is made of a high permeance magnetic material..]. .[.17. Anelectromechanical transducer as defined in claim 14, wherein saidarmature comprises a permanent magnet..].
 18. An electromechanicaltransducer as defined in claim 14, wherein said armature is tubular. 19.An electromechanical transducer as defined in claim 14, wherein saidmeans for mounting said armature comprises a ring of resilient materialsupported along its outer periphery and engaging the armature along itsinner periphery.
 20. An electromechanical transducer as defined in claim14, wherein said yoke means comprises four rods of a high permeancemagnetic material extending substantially parallel to the central axisof the airgap, which rods at one end abut against their respectivemagnetic portions and at the opposite end are magneticallyinterconnected by means of a common cross-piece.
 21. An electromagnetictransducer as defined in claim 13, wherein said pair of permanent magnetportions comprise portions of a cylindrical magnet having a hollowcentral axial portion, said armature being mounted for movement withinsaid hollow central axial portion.